1. Field of the Invention
This invention relates to artificial joints for implantation in the body and in particular to porous-coated artificial joints.
2. Description of the Prior Art
One of the critical problems in the field of artificial joints involves achieving a strong attachment between the patient's bone and the prosthesis both at the time of implantation and throughout the life of the prosthesis. A variety of approaches have been considered to solve this problem. For example, bone cement has been widely used to affix prostheses to bone. This approach in general provides a strong initial bond, but it has been found that the cement is often the first part of the prosthetic reconstruction to fail during use.
Mechanical fixation approaches have also been used. For example, the outside surfaces of prostheses have been provided with projections of various shapes designed to mechanically engage the patient's bone. Similarly, the outside surfaces of prostheses have been threaded, and the prosthesis screwed into the patient's bone to provide the desired mechanical fixation. Bone screws which pass through the prosthesis have also been used to provide fixation. These approaches frequently provide strong initial fixations which endure. However, some such apparently strong initial fixations become loose in time periods varying from a few weeks to a number of years. The reasons for such loosening are not known with certainty. The loosening, however, is an area in which such initially firm mechanical fixations clearly need to be improved.
Improved long-term fixation has been achieved through the use of porous-coated prostheses. In accordance with this approach, all or a part of the outside surface of the prosthesis is coated with small metal balls so as to produce a lattice work of submillimeter-sized interstices into which bone can grow. The bone ingrowth provides the desired long-term fixation. Unfortunately, in some situations, porous-coated prostheses lack good initial fixation.
In an attempt to provide both long-term and initial fixation, prostheses have been designed which incorporate both porous coating and mechanical fixation. Thus, an acetabular cup has been designed by Biomet, Inc., (Warsaw, Indiana), which includes a porous-coated dome and a circumferential, external thread located in the region adjacent the base (widest part or equator) of the cup. Also, Zimmer, Inc., (Warsaw, Indiana), Osteonics Corp. (Allendale, New Jersey), and others have made femoral components for hip prostheses which include 1) smooth, non-porous-coated portions sized to provide a mechanical, interference fit with a shaped cavity formed by the surgeon in the patient's bone, and 2) selected areas of porous coating.
Problems, however, have still remained. For example, the prostheses designed to date have only had limited porous-coated areas, e.g., only the dome of the prosthesis for the Biomet acetabular cup discussed above. Also, the porous-coated areas have been located in places where their effectiveness in providing strong fixation has been limited. Thus, in the case of acetabular cups, because of the anatomy of the pelvis and the moment arms involved, the most effective area for fixation is that area adjacent the base of the cup. Yet, to date, to achieve strong initial fixation, the external thread has been applied to the region of the base of the cup and the porous-coating has been applied only to the less effective dome region.
In the case of femoral stem prostheses, as discussed above, combinations of porous-coated portions together with smooth portions designed to provide a force fit in a shaped cavity in the bone have been used. None of these devices has included the combination of 1) porous-coated areas for long term fixation by bone ingrowth, and 2) means whereby the device could cut its own cavity or partial cavity to achieve a precise, secure, immediate mechanical fixation of the device to the patient's bone.
In view of the foregoing, the full potential of porous-coating as a means to achieve long-term prosthesis stability has not been realized in the prior art.